This invention relates to superconducting compositions, and a method for the preparation thereof. More specifically, this invention is related to superconducting fibers of niobium carbonitride.
It is well known that certain metals, alloys, and compounds go through a superconducting transition into a state in which the electrical resistance has a value approximately 0 at temperatures approaching absolute zero. The temperature at which such a material becomes superconducting is referred to as the transition temperature, T.sub.c. When a superconductive material is subjected to a magnetic field, a current will flow as long as the temperature of the material remains below the transition temperature, T.sub.c, and the magnetic field is below a critical level, H.sub.c2. This critical field is a function of temperature, increasing as temperature is reduced below T.sub.c.
It is desirable that H.sub.c2 be as high as possible, just as it is desirable that T.sub.c be high. For example, in the production of superconductive magnetic coils, a superconductive material is desired which will provide a high critical field. When such a coil is in use, the field to which the coil is subjected must remain below the specified H.sub.c2 of the superconductive material.
The first superconducting coils that were built used wire consisting only of superconducting material. It was found, however, that the performance of such coils was seriously degraded by unpredictable premature transition of the superconductor to the normal (non-superconducting) state. In order to overcome this problem, various forms of composite electrical superconductive materials have been used. These composite conductors comprise a superconductor in intimate contact throughout its length with a normal conductor of high conductivity, the normal conductor acting as a shunt when a transient instability causes a portion of the superconductor to become non-superconductive. When the transient has ended, the current returns to the superconducting portion. The normal conductor utilized is ordinarily copper or aluminum. An example of such a method is demonstrated by U.S. Pat. No. 3,594,226, to Thomas. This patent describes a composite electrical conductor comprising a carbon fiber coated with a superconductor, such as niobium/tin (Nb.sub.3 Sn). The superconductor coating is applied by vapor deposition. Such fibers may be utilized to form a yarn or cable. A more recent description of a composite superconductive body is given by U.S. Pat. No. 3,748,728, of Watson. This reference teaches a porous glass matrix having a granular system of superconductive material disposed within the pores thereof. Adjacent grains of superconductive material are spacially separated, but are electrically connected by electron tunneling. The superconductive material is forced into the pores of the matrix under high pressure. Another composite superconducting structure is taught in U.S. Pat. No. 3,380,935, to Ring. This patent teaches a metal and/or polymer matrix and a superconductor material, in amounts of 20-90 percent by volume of structure, which is in discontinuous fiber form. Another superconducting composite material is disclosed in U.S. Pat. No. 3,447,913, to Yntema. This composite material includes a superconductive matrix in which is embedded solid discrete particles of a non-superconducting, non-conducting material.
Much emphasis in the field of superconductivity has been placed in finding materials with high transition temperature (T.sub.c), high upper critical field (H.sub.c2) and high current density (J.sub.c). The areas of research have included transition metal refractory materials, many of which have been found to possess excellent superconducting properties. Exemplary of such materials are transition metal carbides and nitrides, such as niobium carbide and nitride. U.S. Pat. No. 3,364,099, to Forshey et al, relates to superconducting niobium carbide and nitride products in the shape of fibers, films, and plates. The fibrous niobium carbide of said patent exhibits a significantly higher transition temperature than previously reported bulk niobium carbide. Bulk niobium carbide has been reported as exhibiting a transition temperature no higher than 11.1.degree.K, whereas fibrous niobium carbide was found to exhibit transition temperature as unexpectedly high as 17.3.degree.K. Similarly, fibrous niobium nitride thus prepared exhibited a transition temperature of 17.degree.K as compared with reported transition temperatures of approximately 15.degree.K. This reference also teaches the preparation of fibrous materials containing mixed niobium carbide/nitride crystals. Another teaching of carbide fibers may be found in Wainer et al, U.S. Pat. No. 3,269,802. This reference teaches the preparation of niobium carbide by a reaction of niobium pentachloride with carbonized rayon cloth at elevated temperatures. Dry hydrogen carrier gas is utilized in the formation of the niobium carbide.
The preparation of carbonitride materials per se has been known for some time. For example, Beatty et al, U.S. Pat. No. 3,577,485, teaches the conversion of actinide oxide-carbon particles to actinide carbonitride by contacting the actinide oxide-carbon particles with nitrogen in a fluidized bed furnace at elevated temperatures. A single phase material may be formed by removal of free carbon.